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Abstract
Background New Zealand Māori have some of the highest rates of Group A streptococcal infection (GAS) in the world. GAS elevates titres of antistreptokinase (SK) neutralising antibodies and may induce resistance to SK.
Methods Anti-SK titres were measured in 180 patients presenting with symptoms consistent with an acute coronary syndrome to three New Zealand rural hospitals, selected because they provide care for patients from communities with different socio-economic and ethnic mixes (Māori proportions varying between 6% and 67%).
Findings Compared with the community with the lowest proportion of Māori, patients in the community with the highest proportion of Māori had mean anti-SK titres that were 2.8 times higher (p=0.05). They were 2.5 times more likely to have a high anti-SK titre (33% vs 13% p=0.035).
Interpretation Alternatives to reperfusion with SK should be the first-choice therapy in hospitals serving communities with high rates of GAS such as some predominantly Māori and Pacific Island communities.
- Antibodies
- cardiovascular
- myocardial ischaemia
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Introduction
Ischaemic Heart Disease is the leading cause of death for New Zealand Māori who have age-standardised ischaemic heart disease mortalities two to three times higher than New Zealanders of European descent.1
Māori are also more likely to live in remote rural areas2 where fibrinolytic drugs remain the standard reperfusion therapy for patients presenting with an ST-elevation myocardial infarction (STEMI). Because of the time critical nature of reperfusion therapy, the distances involved and a limited number of interventional cardiology centres, primary percutaneous coronary intervention is unlikely to become a practical alternative for most rural New Zealanders.
Streptokinase (SK) continues to be widely used in rural New Zealand, although in recent years there has been a shift to newer, more expensive, fibrinolytic agents.3 SK is the only widely available fibrinolytic in the Pacific Islands.
SK is a naturally occurring product of Group A streptococcus (GAS). The development of neutralising antibodies against streptokinase (anti-SK antibodies) has been demonstrated after treatment with SK4 5 and after infections with GAS. However, the literature is divided about whether high levels of anti-SK antibody titres confer clinically important SK resistance.6–13 The uncertainty is compounded by the fact that the major treatment trials of SK did not include patients from the developing world where GAS is more likely to be endemic.14 A precautionary approach suggests that SK should be avoided in patients with previous SK exposure or recent GAS infection, or in communities with high endemic levels of GAS, now that alternative agents exist.
The only GAS disease for which global disease burden estimates have been made is acute rheumatic fever/rheumatic heart disease. Although acute rheumatic fever is rare in industrialised countries, the highest reported incidence is found in Māori and Pacific people in New Zealand, Aboriginal Australians and Pacific Islanders.15–20 The incidence varies considerably across New Zealand.16
Urdahl demonstrated elevated anti-SK immunoglobulin G (IgG) levels and SK resistance in a remote Australian Aboriginal community.21 Similar results were obtained in a more recent study that measured anti-SK titres in patients presenting with acute coronary syndromes in another predominantly aboriginal rural Australian community.22 These findings led to a change in Australian guidelines which now recommend that alternatives to SK be used in rural areas, particularly for indigenous patients.23
Anti-SK titres have not previously been measured in populations in either New Zealand or the Pacific Islands despite the known high endemic levels of GAS. No specific recommendations exist about the use of SK in these populations.
Because even a small delay in its administration significantly reduces the effectiveness of fibrinolysis, it is preferable to develop treatment regimes that can be consistently applied across a whole community. An ethnically based genetic predisposition to acute rheumatic fever remains unproven, and other factors such as low socio-economic status and housing are more likely to be responsible.24 This study was therefore designed to detect differences in anti-SK titres between different communities rather than between ethnic groups.
Methods
Three rural hospitals, serving communities that differed ethnically and socio-economically were chosen for the study. Figure 1 shows the location of the rural communities, and table 1 describes their demographic characteristics. Socio-economic status was taken from census reports of community deprivation measured on a 10-point scale from 1=least deprived to 10=most deprived.25 Sixty patients were enrolled in each centre between 1 May and 30 December 2006. Patients presenting with symptoms consistent with an acute coronary syndrome were eligible for inclusion. Patients who had had SK at any time in the past were excluded.
Map of New Zealand showing study locations.
Characteristics of study participants and study communities
After consenting to participate in the study, a venous blood sample was drawn from each study participant and sent for determination of antistreptolysin O (ASOT) and anti-SK titres. ASOT were measured by Southern Community Laboratories in Christchurch using a standard commercially available kit. Because commercial kits to measure anti-SK AB titres are no longer available, the Department of Microbiology & Immunology (Otago School of Medical Sciences, Dunedin) developed the following ELISA assay and undertook testing. Nunc-immuno plate F96 Maxisorp (Cat. 442404) were coated with 100 μl/well of 1 μg/ml pure SK without albumin (a gift from Behring, Switzerland) in Dulbecco phosphate-buffered saline (PBS; Cat. 21600-010 m, Invitrogen). Pure SK was utilised, since carrier proteins in streptokinase preparations might also interfere with the assay if natural or induced (anti-bovine) albumin antibodies are present in patient sera. Plates were incubated for 1 h at 37°C, 5% CO2. The plates were washed three times with 0.05% Tween 20 molecular biology grade (BDH Cat. 437082Q)/PBS. Plates were blocked with 200 μl/well of 0.1% sodium caseinate (AroTec Diagnostics, Wellington)/PBS for 10 min at room temperature. The blocking solution was removed, and serum samples diluted 1/200 in 0.1% sodium caseinate/PBS were added at 100 μl/well. Plates were washed, and a 1/3000 dilution (in 0.1% sodium caseinate/PBS) of goat anti-human IgG Fc (Sigma A-0170) added. The plates were incubated for 1 h at 37°C, 5% CO2 and then washed three times with 0.05% Tween20/PBS before development with 100 μl/well TMB Single Solution (ZYMED Cat. 00-2023). The reaction was stopped with 50 μl/well 2 N H2SO4. Plates were then read at 450 nm. Unit values were calculated against a standard curve for each of the samples using Graph Pad prism. The ELISA was first validated on 10 patients from each locality using SK-coated and blank (uncoated) wells. These sera showed minimal, non-specific reaction uncoated wells, demonstrating the lack of non-specific reaction of our ELISA. To ensure that reactivity could be calculated on a linear basis, a patient displaying high antibody reactivity against streptokinase was arbitrarily assigned as standard with the unit value (1 000 000 anti-streptokinase Units/ml). The standards were serially diluted from 1×104 U/ml in 1:1 dilutions with caseinate/PBS. All other patient samples were tested against this sera, and unit values calculated from OD readings using a standard curve generated from this index patient.
Most clinical interest is on patients with high anti-SK titres who may have clinically important SK resistance. In the absence of agreement on the point at which anti-SK titres become clinically important, a ‘high’ titre was defined as >100×103. This was the top quintile (34 measures in total) and included all outlying results.
The sample size was large enough to identify a 17% difference between the three study centres in the number of high titres, with 80% probability. Smaller differences would be unlikely to justify communities adopting different fibrinolytic strategies.
Ethics approval was obtained from the New Zealand Multi-region Ethics Committee and consultation was undertaken with Māori representatives in each of the study localities. Informed consent was obtained from all participants.
Results
Six patients were excluded because of previous exposure to SK. There were statistically significant differences between sites in proportions of Māori and non-Māori participants (p<0.001), and in age between ethnic groups overall (p=0.001) but not between sites (p=0.095).
The mean anti-SK antibody titre at the Hokianga site was 2.8 times higher than the Dunstan site (168006 vs 60082, p=0.05). Patients from the predominantly Māori community of the Hokianga were 2.5 times more likely to have a high titre than those from the predominantly European community of Dunstan (32.8% vs 12.7% p=0.018). Differences between Thames and both Hokianga and Dunstan were not significant.
The absence of Māori patients at the Dunstan site makes it impossible to examine differences between ethnic groups free of the confounding effect of location. Overall, however, Maori had mean anti-SK antibody titres that were twice that of non-Māori (171 469 and 88 016 respectively; p=0.032), and Māori were 1.8 times more likely to have a high titre than non-Māori (29% vs 16% p=0.05). Anti-SK titres are plotted against ASOT in figure 2 (log scale). Over the range of the data, the correlation is 0.662 (p<1e–15), which drops to 0.415 (p=0.007) for data above the high anti-SK line.
Antistreptokinase (antiSK) and antistreptolysin O (ASOT) levels for Maori and non-Maori study participants.
Discussion
The study sites were chosen because of the different ethnic and socio-economic mix of each community. We found that patients in the community that was predominantly Maori and with a lower socio-economic status (Hokianga) were significantly more likely to have a high anti-SK titre than the community that was predominantly European and had a higher socioecomonic status (Dunstan). The more ethnically and socio-economically mixed Thames community fell between the other two communities.
The differences we found were less marked than in the Australian study in which the mean anti-SK titre of the rural indigenous community was almost 20 times higher than the non-indigenous urban community.23 This is consistent with the very high incidence of acute rheumatic fever among Australian aboriginals; at least three times higher than even New Zealand Māori and the highest reported in the world.26
Significant differences in ASOT levels were not identified between sites. The positive correlation between ASOT and anti-SK AB levels is weaker when the analysis is confined to only the high titre results, and this study does not provide support for the use of ASOT as a surrogate for anti-SK titres at high titres as has been suggested by others.24
While it is it not possible to determine the extent to which elevated titres translate into clinically important SK resistance, it seems prudent to avoid the use of SK in those with high anti-SK titres when an alternative fibrinolytic agent is available. These results therefore provide support for the recent trend to move away from SK to newer non-immunogenic fibrinolytic agents in rural New Zealand. This is particularly relevant for predominantly Maori communities.
This study is another reminder of the need to exercise caution when translating to other ethnic groups the results of drugs trials completed with mainly Caucasian populations, particularly when a mechanism that could confer drug resistance has been identified. Issues remain for treating Pacific Islands patients with ischaemic heart disease. Pacific Islanders living in New Zealand have an incidence of acute rheumatic fever that is even higher than that of Maori (1.5 to 2 times).16 Polynesians living in their Pacific Island homelands of Samoa, Cook Islands, French Polynesia and Tonga have among the highest documented prevalence of rheumatic heart disease in the world.17 18 Pacific Island countries are experiencing rising rates of diabetes and ischaemic heart disease but in most places lack facilities for urgent percutaneous coronary intervention and continue to rely on SK as their sole fibrinolytic agent. Unfortunately, alternatives to SK are considerably more expensive and may be beyond the limited health budgets of many of these Pacific countries despite doubts about the efficacy of SK in these communities. Further research is needed in these communities.
Take-home messages
A patient presenting with an acute coronary syndrome who lives in a New Zealand rural community that is predominantly Maori is significantly more likely to have a high anti-streptokinase antibody titre.
The newer non-immunogenic fibrinolytic agents should be used in preference to streptokinase in the management of patients with acute myocardial infarction in these communities.
It is likely that similar issues exist in many Pacific Island communities. The use of newer fibrinolytic agents in these communities would have major cost implications. Further research is therefore needed.
Acknowledgments
I Norman is thanked for coordinating the study.
References
Footnotes
Funding The Heart Foundation; The Healthcare Otago Charitable Trust.
Competing interests None.
Ethics approval Ethics approval was provided by the New Zealand Multi-Region Ethics Committee, New Zealand Ministry of Health.
Provenance and peer review Not commissioned; externally peer reviewed.